Issue 29

A. Fortini et alii, Frattura ed Integrità Strutturale, 29 (2014) 74-84; DOI: 10.3221/IGF-ESIS.29.08 75 biocompatibility. In particular, the near-equiatomic NiTi alloys are the most important practical shape memory alloys, extensively used for an increasing number of applications in different fields of engineering. The NiTi alloy system, in addition to the one-way shape memory effect (OWSME) may also show the two-way shape memory effect (TWSME). Through the SME the material could recover the macroscopic shape of the austenitic parent phase upon heating above A f , while through the TWSME, it also exhibits a return to the reoriented martensitic shape upon cooling below M f , in the absence of applied stress. This spontaneous repeatable shape change on heating and cooling is an acquired behaviour, rather than an inherent property of the material, obtained by specific thermomechanical loading cycles, named training treatments, to which the SMA has been subjected [1, 2]. This procedure develops a residual internal stress state which guides the growth of certain martensitic variants, towards the preferred orientations, regarding the deformation adopted during training, when SMA is stress-free cooled [3]. From a crystallographic point of view, it is widely accepted that preferential martensite formation is due to the generation of permanent defects in the parent phase resulting from training procedure [2, 4]. As a result the material will change its shape as it changes its phase. Different training methods for obtaining a TWSME are described in literature and the influence of the type and training parameters, the stability of the TWSME during thermal cycling as well as the efficiency of the methods have been widely investigated [1, 2, 5-10]. Generally, training procedures have the purpose to induce the low temperature shape in the sample introducing permanent defects such as dislocations, stabilised stress induced martensite and precipitates [4, 5, 11, 12]. Common methods of training include: shape memory cycling, pseudoelastic cycling, combined shape memory cycling/pseudoelastic cycling and constrained cycling of deformed martensite [1, 5, 13, 14]. All of these thermomechanical treatments deal with the repetition of a procedure that considers the transformation from austenite to a preferentially oriented martensite or from deformed martensite to austenite [13]. Due to their attractive behaviour, NiTi shape memory alloys have been successfully applied in a broad set of innovative applications in aerospace, biomedical, mechanical and civil engineering fields. In particular, SMAs are excellent materials to be used as actuating elements in smart structures given that they could generate large force and displacement during the phase transformation. Many active deformable structures, in which NiTi strips or wires are embedded in a polymeric matrix, are based on the OWSME and, as a consequence, an external force is required to bring back the structure to its original shape. To this end, the application of the TWSME on the SMA elements plays an important role since it make possible to achieve more compact and simple configuration systems with improved performances upon demand, thanks to the integration of multiple functions in a single component. A TWSME behaviour in bending gives the macroscopic reversible shape change of the functional structure upon heating and cooling, without demanding the recovery to the elasticity of the polymeric structure. The SME behaviour in bending has been experimentally and theoretically investigated by many authors. The works [15, 16] have focused on the bending properties of polymer-SMA composite microdevices, for example, microvalves. In these actuators, the right combination of the polymer and SMA thin film leads to a two-way stable behaviour during heating and cooling. More recently, Roh and Bae [17] have numerically and experimentally investigated the thermo-mechanical behaviour of NiTi strips associated with stress and temperature-induced transformations. The activation of the SMA strips causes a bending deformation on the actuator, which has remarkable vertical tip deflections. Larger deflections can be obtained by increasing the initial strain of the SMA. Irzhak et al. [18] observed giant reversible bending deformation at a sub-micrometre scale by using a nichel-SMA composite strip. Furthermore, some authors focused on the effects of the pre-strain, recovery temperature and bending deformation on the shape memory effects [9, 19-24]. To predict the one- dimensional thermomechanical behaviour of SMA, several macroscopic constitutive models are currently available in literature, an overview can be found in [3, 23, 25-28]. It is widely known that the response of SMA is dependent on stress and temperature fields and it is closely connected with the crystallographic phase of the material and the thermodynamics underlying the transformation processes. While some studies are focused on the two-way shape memory effect exhibited by bent wires [9, 13, 19], the present study is aimed at investigating the stability of the TWSME behaviour induced on near-equiatomic NiTi strips by means of the so-called shape memory cycling method, which here is applied to bending deformations. In particular, the possibility to take advantage of the TWSME is here considered in order to optimise the behaviour of strips embedded in active deformable structures. To this end, a strip is firstly subjected to a specific thermomechanical treatment, in order to memorise a bent shape with a uniform curvature, and then trained to realise the spontaneous recovery to the martensitic shape upon cooling. In particular, the training process basically consists of the repetition of the following steps: i) cooling the SMA to below M f to form martensite, ii) deforming to the desired cold shape, below the shape memory limit, iii) heating in stress-free condition to above A f to recover the original high temperature shape [13, 14]. It should be highlighted that the amount of spontaneous shape change on cooling is significantly less than those being induced in the ii) deformation step and it is typically between 0.2 and 0.25% of the training strain value [4, 14]. After the training